Electronic pilot ignition and flame detection circuit

ABSTRACT

An electronic pilot ignition and flame detection circuit for use in a fuel ignition system including a spark ignition circuit operable when energized to generate sparks for igniting gas emanating from a pilot source, a switching circuit including a normally de-energized relay and a normally non-conducting silicon controlled rectifier which controls the relay, and a pilot flame sensing circuit operable as a pulse generating circuit for sensing the pilot flame and providing pulses of a first amplitude for enabling the silicon controlled rectifier to effect energization of the relay whenever the pilot gas is ignited to cause the spark ignition circuit to be de-energized and to prepare an energizing path for a main burner gas valve solenoid. The flame sensing circuit provides pulses of a lower amplitude whenever the pilot flame is extinguished to preclude enabling of the silicon controlled rectifier, such that the relay is disabled causing the energizing path for the gas valve solenoid to be interrupted and the spark ignition circuit to be re-energized. Alternatively, the gas valve solenoid may be substituted in the circuit for the relay coil to be controlled directly by the silicon controlled rectifier. In such a case a separate self extinguishing pilot relight circuit controls the pilot ignition.

United States Patent Matthews Sept. 2, 1975 ELECTRONIC PILOT IGNITIONAND FLAME DETECTION CIRCUIT [75] Inventor: Russell B. Matthews, Goshen,Ind.

[73] Assignee: Johnson Service Company,

Milwaukee, Wis.

22 Filed: Dec. 7, 1973 21 Appl. No.: 422,693

[52] US. Cl. 431/46; 431/51; 431/59;

Primary Examiner-Carroll B. Dority, Jr. Attorney, Agent, orFirm-Johnson, Dienner, Emrich & Wagner [5 7] ABSTRACT An electronicpilot ignition and flame detection circuit for use in a fuel ignitionsystem including a spark ignition circuit operable when energized togenerate sparks for igniting gas emanating from a pilot source, aswitching circuit including a normally de-energized relay and a normallynon-conducting silicon controlled rectifier which controls the relay,and a pilot flame sensing circuit operable as a pulse generating circuitfor sensing the pilot flame and providing pulses of a first amplitudefor enabling the silicon controlled rectifier to effect energization ofthe relay whenever the pilot gas is ignited to cause the spark ignitioncircuit to be de-energized and to prepare an energizing path for a mainburner gas valve solenoid. The flame sensing circuit provides pulses ofa lower amplitude whenever the pilot flame is extinguished to precludeenabling of the silicon controlled rectifier, such that the relay isdisabled causing the energizing path for the gas valve solenoid to beinterrupted and the spark ignition circuit to be re-energized.Alternatively, the gas valve solenoid may be substituted in the circuitfor the relay coil to be controlled directly by the silicon controlledrectifier. In such a case a separate self extinguishing pilot relightcircuit controls the pilot ignition.

23 Claims, 4 Drawing Figures I -l20 V.

PATENTED SEP 2 I975 sum 3 OF 9 02w QZ/J.

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PATENTEDSEP' 2M5 ELECTRONIC PILOT IGNITION AND FLAME DETECTION CIRCUITBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to fuel ignition systems, and more particularly to anelectronic pilot ignition and flame detection circuit for use in suchsystems to monitor a pilot flame and effect the de-energization of a gasvalve in response to failure of the pilot flame or a component failurein the pilot ignition and flame detection circuit.

2. Description of the Prior Art Many known fuel ignition systems employa thermocouple device to monitor a standing pilot flame and a relaycontrolled by the thermocouple device to effect de-energization of a gasvalve, for example, to shut down the system in the event of a pilotflame failure. The thermocouple used to sense the pilot flame in suchsystems has a response time of approximately 20-45 seconds before thecontrol relay will be de-energized to effect closing of the main gasvalve. It is desirable to eliminate a standing pilot flame to conservegas and at the same time provide a fast response time of the controlarrangement therefor.

A fast response time makes possible the elimination of standing pilots,a very desirable function considering the impending gas shortage and thevast amount of gas consumed by standing pilots.

In order to accomplish this and still retain the inherent safety of apilot ignition of a main burner, it becomes necessary to turn the pilotoff when the thermostat is not calling for heat. This means that anytime the thermostat calls for heat the pilot burner must be ignitedfirst, second the presence of the pilot flame verified and then the mainburner turned on.

The response of this system is so fast that less than I second isrequired for the complete sequence. This means that the circuit canreplace a standing pilot system without any detectable difference inperformance to the heating system.

SUMMARY OF THE INVENTION The present invention has provided anelectronic pilot ignition and flame detection circuit including anelectronic flame sensing circuit which has a much faster response timethan prior art systems employing a thermocouple-relay combination. Theresponse time of the electronic flame sensing circuit may, for example,be one second or less thereby eliminating the need for a gas-wastingstanding pilot flame without otherwise effecting the heating system.

Moreover, the flame detection circuit of the present invention mayinclude an electronic pilot ignitor controlled by the electronic flamesensing circuit such that the generation of ignition sparks for ignitingthe pilot is provided automatically whenever loss of pilot flame isdetected. Also, the electronic flame detection circuit of the presentinvention is extremely fail-safe in that any given component failure foran open-circuit condition to a short-circuit condition will not resultin an unsafe condition in which the main gas valve may be energized whenthe pilot flame is not present.

In accordance with an exemplary embodiment of the invention, theelectronic pilot ignition and flame sensing circuit includes pilotignition means operable when energized to generate ignition sparks forigniting pilot gas emanating from a pilot source to establish a pilotflame.

A switching means is normally de-energized when the pilot flame isextinguished to extend a cyclical AC signal supplied to the circuit overinput means and a pair of input conductors to the ignition means forenergizing the ignition means. The switching means is operable to effectthe de-energization of the ignition means and to prepare an energizingpath for gas valve means which supplies gas to gas burner apparatuswhenever the pilot flame is lit.

A flame sensing means operable as a pulse generating circuit providespulses at first and second levels for controlling the de-energizationand the energization of the switching means as a function of thepresence or absence of the pilot flame. The flame sensing means includesa controlled switching device having a pair of control electrodes, afirst circuit means including first capacitor means connected between afirst one of the control electrodes and one of the input conductors, andsecond circuit means including second capacitor means connected betweenthe second control electrode and the one input conductor.

The flame sensing means further includes means connected between theinput conductors for providing a charging path for the first capacitormeans to permit the first capacitor means to charge to a predeterminedvalue during each first half cycle. of the AC signal to provide a firstpotential at said first control electrode, and means for providing asecond capacitor means to charge to a predetermined value at the firstrate whenever the pilot flame is extinguished and to charge to saidpredetermined voltage at a faster rate whenever the pilot flame isestablished to provide a second potential at said second controlelectrode.

The controlled switching device is rendered conductive at a time duringeach first half'cycle of the AC signal when the potential differencebetween the first and second control electrodes exceeds a predeterminedvalue, permitting the first capacitor means to discharge over thecontrolled switching device to effect generation of pulses at the firstlevel for energizing the switching means whenever a pilot flame isestablished and to effect the generation of pulses at the second levelcausing de-energization of the switching means whenever the pilot flameis extinguished.

In addition, in the eventof a component failure in the pilot ignitionand flame detection circuit, such as in the flame sensing means, forexample, the controlled switching device will be maintainednon-conductive so as to disable switching means and interrupt theenergizing path for the main gas valve solenoid.

Thus, the electronic pilot ignition and flame detection circuit of thepresent invention has three levels of operation. For the condition of apilot flame failure, the system is operable at a first voltage level inwhich the pulses provided by the flame sensing circuit are ineffectiveto enable the switching means and accordingly the energizing path forthe main gas valve solenoid is interrupted. Moreover, the pilot ignitionmeans will be energized to effect re-ignition of the pilot flame.

The presence of a pilot flame places the system at a second operatinglevel in which the flame sensing circuit is operable to provide pulsesfor maintaining the switching means enabled permitting the main gasvalve solenoid to be energized. In the case of a component failure, thesystem is operable at a third level in which the flame sensing circuitis disabled such that the switching means will remain de-energizedwhether or not the pilot flame is present.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of apreferred embodiment of the invention operable from an alternatingcurrent source and using a relay energized by a silicon controlledrectifier for controlling main burner valve operation;

FIG. 2 is a schematic circuit diagram of another embodiment of theinvention, but substituting a solenoid of the main valve for the relaycoil of FIG. 1, thereby controlling the main burner valve directly bythe silicon controlled rectifier and utilizing an ignitor circuit whichis self-extinguishing;

FIG. 3 is a schematic circuit diagram of still another embodiment of theinvention similar to FIG. 2 in that the main valve solenoid iscontrolled directly by the silicon controlled rectifier but operablefrom a DC source and using an ignitor which is controlled by means of asecond coil wound on the solenoid of the main valve; and

FIG. 4 is a schematic circuit diagram of yet another embodiment of theinvention, similar to FIG. 1 in that a relay is responsive to a siliconcontrolled rectifier to control the main valve, but is energized from aDC.

source.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, there isshown an exemplary embodiment for an electronic pilot ignition and flamedetection circuit provided by the present invention. The electronicflame detection circuit 10, which may be employed in a fuel ignitionsystem, includes an electronic pilot ignition circuit 11 which suppliesvoltage pulses derived from an AC source to a pair of ignitionelectrodes 12 and 13 to ignite a gaseous fuel emanating from a pilotsource 14 of a gas burner apparatus (not shown).

Energizing power for the flame detection circuit 10, including the pilotignition circuit portion 11, is supplied over an input transformer T2which has a primary winding 15 connectible to an AC voltage sourcewhich,

for example, may be a standard 60 Hertz, I volt A.C.

line voltage source, and a secondary winding 15A. The input transformerT2 is a step-down transformer which provides approximately 24 volts A.C.between terminals 40 and 41 of winding 15A when the primary winding 15of the transformer T2 is connected to a 120 volt A.C. source.

Terminal 40 of the secondary winding 15A of the input transformer T2 isconnected through normally open TI-IS thermostatic contacts over aconductor L3 and normally closed contacts 21A of a relay 21 to a firstterminal 22 of the ignition circuit 11. Terminal 41 of the secondarywinding 15 of the input transformer T2 is connected over a conductor L4to a second terminal 24 of the ignition circuit 11.

The ignition circuit 1 1 is more fully described in United States patentapplication Ser. No. 307,077 filed Nov. 16, 1972, now US. Pat. No.3,806,305 and has an output winding 25 having a first terminal 26connected to one of the ignition electrodes 12 and a second terminal 27connected to the other ignition electrode 13.

Accordingly, when the thermostat calls for heat THS contacts close andpower is applied to the flame detecting circuit 10 over the inputtransformer T2, AC current flows through the ignition circuit 11producing voltage pulses across the output winding 25 of the ignitiontransformer T1, which pulses are applied to the electrodes 12 and 13,producing sparks between the ignition electrodes 12 and 13 for ignitinggas emanating from the pilot source 14. W'hen power is applied to theignitor circuit 11 from transformer T2 on the first half wave of appliedvoltage and with terminal 40 assumed positive with respect to 41,current flows through thermostat contacts TI-IS over line L3 throughnormally closed relay contacts 21A through capacitor C10, resistor R20,capacitor C11, diode D10 to terminal 24, charging capacitors C10 andC11. On the next half cycle or reverse polarity, current flows fromterminal 41, terminal 24, diode D11,.capacitor C10, normally closedcontacts 21A and thermostat contacts Tl-IS. This circuit acts as avoltage doubler, substantially doubling the charge of capacitor C11,which charge is applied across the anode-cathode circuit SCR T1. As thisvoltage doubling effect occurs, the bias across diode D10 of SCR T1 issufficient to cause SCR to fire across its anode-cathode circuit causingcapacitor C11 to discharge through the anode-cathode circuit and throughthe primary winding 23 of ignition transformer T1. This pulses thesecondary 25 of the transformer causing a high voltage spark acrosselectrodes 13 12 to attempt ignition of the pilot gas coming from pilotburner 14. This occurs once every cycle of the applied voltage until thepilot gas is ignited.

The flame detection circuit 10 also includes a flame sensing circuit,indicated generally at 30, which is supplied with approximately voltsA.C. through isolating step-up transformer T3. Circuit 30 is operable toprovide a pulse output indicative of the presence or absence of thepilot flame. The flame sensing circuit 30 includes a flame sensingelectrode 31 and a controlled switching device 32, embodied as aprogrammable unijunction transistor (PUT), such as the type 2N6028commercially available from Motorola. The flame sensing circuit 30 alsoincludes an anode, a control network 33 and a gate control network 34for the PUT device 32.

The flame sensing electrode 31 is connected over a resistor R1 toconductor L1 and is positioned in a spaced-relationship with a groundreference point 35 for the electronic flame detection circuit 10,normally providing a high resistance path between conductor L1 and thereference point 35. The ground reference point 35 may, for example, be ametallic ground provided by a gas burner apparatus or the pilot source14. The flame sensing electrode 31 is located in the region in which thepilot flame is to be produced such that the pilot flame will bridge thegap 36 between the electrode 31 and the reference point 35 therebylowering the re sistance of the current path over the electrode 31between conductor L1 and the reference point 35 whenever the pilot flameis present.

The gate control network 34 determines the gate potential forthenormally non-conducting PUT device 32. The gate control networkincludes a capacitor C1 which is connected between the reference point35 and conductor L2. Capacitor Cl will charge at a first rate wheneverthe pilot flame is unlit. However, whenever the pilot flame bridges thegap 36 between the sensing electrode 31 and the reference point 35, theresistance of the charging path for capacitor C1 will be lower andcapacitor C1 will charge at a faster rate.

The gate control network 34 further includes a re's'istor R2 which isconnected between the reference point 35 and the gate electrode of thePUT device 32, and a resistor R3 which is connected "between the gateelectrode of the PUT device and conductor L2. Resistors R2 and R3 form ableeder path for capacitor C1. A second capacitor C1 "is connectedredundantly in parallel with the capacitor C1 forsafety purposes.

In addition, a resistor R3 and a transistor 50 are connected betweenconductor L2 and the gate electrode of the PUT devi'ce 32"forming aclamping circuit 'to limit the voltage swing at the gate to apredetermined prised of resistors R5, and R7 which are serially.connected between the conductors L1 and L2 with the junction of "theresistors'R5 and R6'at point 37 being connected to the a node electroldeof the PUT device 32 and thus to tinged ca pac'itor c2. Accordingly, acharging path is provided for capacitor C2 from conductor L1 ove'rresistor R5 andc apacitor C2 to conduc- The PUT device 32 is normallynon-conducting and is rendered conductive whenever the'potential' at theanode electrodeek'ceeds the potential at the gateelectrodebyapproximately 0.6 volts as'determined by the action of theanodecontrol network 33 and the gate control network 34, I i

Whenever the PUT device 32 is rendered conductive, a discharge pathisprovided'for capacitor C2 over the anode-cathode circuit of the PUTdevice 32 which supplies pulses provided by the flame sensing circuit Ito a control electrode of a second controlled switching device 39,embodied as a silicon controlled rectifier, which may be the-type ClooAmanufactured by General Electric Co.

The normally I nonconducting silicon controlled rectifier SCR) 39 hasarranode-cathode circuit connected in series with a coilof relay 21between conductors L1 and L2. Th e :co ntrol electrode or gate of theSCR 39 is connectedover the resistor 38' to the conductor L2, aredundant resistor 38 being connected in parallel with the resistor 38for safety purposes.

Relay 21 may comprise a DC relay having a coil resistance ofapproximately 2.5K ohms so that in the case of a short circuit conditionfor the SCR 39, current flowing through the coil 21 will generate a highimpe 7 mally closed contacts 21A'wliic h are connectedlin series withthe normally oper THS contacts and conductor L3 between terminal 22 ofthe ignition circuit 11 Relay 21, njqrm'allylde-e nergized, has nor-"'and terminal 40 of the secondary winding 15A of the input transformerT2. Relay 21 has a pair of normally open contacts 213 which are alsoconnected in series with normally open thermostat switch contacts THSand a' gas valve solenoid 45 which controls the flow of tion sparksbetween the ignition electrodes 12 and 13.

- -When the pilot gas is lit, the SCR 39 will be rendered conductive bythe pulse output of the flame sensing circuit-30 effecting energizationof relay 21. As relay 21 operates, contacts 21A will be opened therebyterminating the generation of ignition sparks at the pilot ignitorn, andcontacts 21B will be closed energizing the main'bumer gas valvesolenoid.

, OPERATION OF THE FLAME DETECTlON l CIRCUIT For purposes ofillustration of operation of the pilot ignition and flame detectioncircuit 10, it is assumed that the circuit 10 is initially unenergizedand that the SCR 39 and the PUT device 32 are cut-off and relay 21 isde-energized. When power is applied to the primary winding 15 of theinput transformer T2, 24 volts AC will be produced across conductors L3and L4, if the Tl-IS contacts are closed causing AC current to flowthrough the ignition circuit 11 and the pilot valve 47 to cause gas toflow from the pilot source 14. Accord- .ingly, voltage pulses will beinduced in the output windignited, a pilot flame will bridge the gap 36between the sensing electrode 31 and the reference point 35.

The proper phase relationship between the pilot ignition circuit 11 andflame sensing circuit 30 is obtained by connecting transformer T3windings such that terminals 19 and 24 are both positive at the sametime.

This phases the circuit 10 so that the spark is at the igni'tiorlelectrodes 12 and 13 when the potential at conductor 22 and conductor Llare both negative and therefore the flame sensing circuit 30 is notsensing.

Accordingly, during a first half cycle of the AC line voltage appliedbetween conductors L1 and L2 when conductor L1 swings positive relativeto conductor L2, current will flow from conductor L1 through resistor R1sensing electrode 31 and the pilot flame to the reference point 35, andover capacitor C1 to conductor L2, permitting capacitor C1 to charge.The voltage across capacitor C1, which is connected over resistor R2 tothe gate electrode of the PUT device 32, establishes a gate potentialfor the PUT device 32.

During the same half cycle, capacitor C2 is charged ov era pathextending from conductor Ll over resistor R5 and capacitor C2 toconductor L2, establishing a potential at the anode of the PUT device32.

The values of capacitors C1 and C2 are selected such that some timebefore the peak of the AC line voltage during the first half cycle ofthe AC line signal the When the SCR 39 is rendered conductive, anenergizing path is completed from conductors L1 and L2 for relay 21which then operates opening contacts 21a to inhibit further sparkingbetween the ignition electrodes l2 and 13 of the ignition circuit 11,and closing contacts 21b to energize the main gas valve. solenoidAccordingly, once the pilot flame has been established and bridges thegap 36 between the sensing electrode 31 and the reference point 35, theaction of the flame sensing circuit 30 will be effective to provideenabling pulses to the gate of the SCR 39 during alternate half cyclesof the applied AC line signal. During the next half cycle of the AC linesignal, when conductor L2 swings positive relative to conductor Ll, theSCR device 39 will be cut off. However, relay 21, once energized, willbe maintained energized by capacitor C3 during the portion of the halfcycle of the line voltage in which the SCR 39 is non-conductive. Theabove conditions will occur every cycle when a pilot flame is prescm atthe sensing electrode 31.

It should be understood that the onlytime pulses will be passed to thePUT device 32 and the gate of the SCR 39 is when the voltage at theanode of the PUT device 32 exceeds that of the gate by plus 0.6 voltsand the SCR 39 will be enabled only when the capacitor C2 has chargedsufficiently to provide the pulse energy required to render the SCR 39conductive.

For the condition when the pilot flame is extinguished, a high impedancepath will be provided over the sensing electrode 31 to reference point35 such that capacitor C1 will have a longer charging time. Accordingly,as capacitors C1 and C2 are charged, the voltage at the gate of the PUTdevice 32 will be lower relative to the voltage at the anode sincecapacitor C2 will be charged at a faster rate over the voltage dividerpath provided by resistors R5, R6 and R7. Consequently, the anodevoltage will exceed the gate voltage early in the half cycle of the ACline signal before capacitor C2 is fully charged. Charge on capacitor C2is limited by voltage on capacitor C1 which is very low when pilot flameis extinguished and less than that required to trigger the SCR 39 intoconduction. Accordingly, whenever the pilot flame is not present, pulsesprovided by the flame sensing circuit 30 will be ineffective to enableSCR 39 to cause relay 21 to be energized.

When the pilot gas is ignited and relay 21 is operated, the gas valvesolenoid 45 will be energized permitting gas to flow to the main gasburner for ignition by the pilot flame. When the main burner flame islit, a current path is provided through the pilot flame and the mainburner flame to the ground reference point 35. Consequently, theresistance between sensing electrode 31 and reference point 35 willdecrease effecting a further increase in the charging current forcapacitor When the main gas burner is lit, the clamping circuit,including the transistor 50 and resistor R3 limits the amplitude of thevoltage provided at the gate electrode of the PUT device 32 to a desiredoperating range which may, for example, be 1 to 4 volts. Accordingly,

with the gate electrode being clamped at a predetermined voltage level,the potential at the anode electrode as provided by the charging ofcapacitor C2 will be capable of exceeding the gate potential by anamount sufficient to trigger the PUT device 32 into conduction andprovide pulses for maintaining the relay 21 energized. If desired, theclamping circuit may comprise alternatively a Zener diode in series witha resistance. i

The electronic pilot ignition and flame detection circuit 10 is alsocharacterized by a fail safe feature by maintaining the proper magnitudeand phase relationship between the voltages that are applied to the gateand the anode of the PUT device 32 in the normal operating mode. Thenormal operating voltage range is l to 4 volts for voltage levels at theanode or gate electrodes of the PUT device 32. For values above this, asmay be caused by a component failure, for example, the anode voltagewill not exceed the gate voltage and accordingly the PUT device 32 willnot conduct0n the other hand, for voltage values below the operatingrange, the anode voltage will exceed the gate voltage before the chargeon capacitor C2 is sufficient to pulse the gate of the SCR 39.

Thus the electronic pilot ignitor and flame sensing circuit 10 may beconsidered as a pulsing system wherein the flame sensing circuit 30 is apulse generator that stops generating pulses for any component failureor flame-out condition. A

The pilot flame which bridges the gap 36 between the sensing electrode31 and the reference point 35 serves as both a resistance and arectifier, and the flame sensing circuit 30 utilizes the rectificationproperties of the flame to maintain the charge on capacitor C1 within adesired operating range. Therefore, any value of resistance between thesensing electrode 31 to the refer- Resistors 38 and 38 2-220Aohms inparallel Resistor RI 470K ohms Resistor R2 2.2 Megohms Resistor R3 4.7Megohms Resistor R7 5.6K ohms Resistor R5 270K ohms Resistor R6 l8K ohms2-0.047 Microfarad in parallel 0.47 microiarad 22 microfarads CapacitorsCl and Cl Capacitor C2 Capacitor C3 Transistor 5t) It is to beunderstood that the operating voltage range which establishes therelationship between the gate and anode voltages, both'phase andmagnitude, for the PUT device 32 is a matter of choice and that thereare a large number of combinations of values for the resistors R and R6,capacitors C1 and C2, resistor R3 and the transistor 50 that can beadjusted to provide satisfactory operation of the circuit.

With reference to FIG. 2, the electronic pilot ignition and flamedetection circuit B is similar in structure and operation to that of thecircuit of FIG. 1 with the exception that the solenoid coil 21 of themain burner valve 45 is substituted in the anode circuit of SCR 39 inplace of the relay 21 of FIG. 1. In addition, a different pilotrelighter circuit is utilized which is selfextinguishing and which isdesignated as 11A. This pilot relighter circuit is more fully describedand claimed in copending application of the same assignee and of whichMatthews is co-inventor, filed Nov. 16, 1972, as Ser. No. 307,077, nowUS. Pat. No.

When thermostat contacts THS are closed to energize the pilot valve toprovide gas for ignition,.igniter 1 1A provides a spark acrosselectrodes 12 and 13 to ignite the pilot gas. As is described in thepreviously aforementioned, US. Pat. No. 3,806,305 a ,spark is producedonce every cycle of applied voltage when the SCR 15 conducts dischargingcapacitor C11 through the primary 25 of ignition transformer T1. Theignitor circuit is self-extinguishing. When its flame sensing portiondetects flame, current flows across electrode 13 to pilot l4 and thenceto ground, as is described in the copending application. This shorts thegate to cathode electrodes of SCR 15 of the ignitor circuit causingpulsing of the ignition transformer T1 to cease. Should the pilot flamebe extinguished, the ignitor circuit automatically reapplies a pulseacross electrodes 12, 13.

When SCR 39 conducts through its anode-cathode circuit as was previouslydescribed for FIG. 1, main valve 45 is energized directly through itssolenoid coil 21 in the anode circuit of SCR 39.

With reference to FIG. 3, the electronic pilot ignition flame detectioncircuit 10C shown is energized from a l2-volt D.C. source (not shown)instead of the 120-volt A.C. source (not shown) of the previous FIGS.land 2. The circuit 10C, however, operates substantially the same asthat described for FIG. 2 where the solenoid of the main valve 45 isactuated directly by SCR 39. The circuit operation differs in thefollowing respects: the pilot valve 110 instead of being operateddirectly through thermostat contacts TI-IS as described for FIG. 2,operates through normally closed contacts 112 of a warp switch connectedin series with the ignitor circuit 11B. As the ignitor circuit attemptsto ignite the pilot gas as was previously described for FIG. 2, currentflows through warp switch heater 111 heating the warp switch, which isof a conventional type. Should the ignitor circuit continue to drawcurrent for more than a predetermined selected time, the warp switchheater 111 is heated sufficiently to energize and open its normallyclosed contacts 112, terminating energization of pilot valve 110 and theignitor 11B. Pilot valve 110 then closes, stopping the flow of gas tothe pilot burner. In this case, the ignitor and pilot valve are inlockout position and must be manually reset. This is done by manualactuation of the warp switch into its normal position in which contacts1 12 are reclosed, placing ignitor 11B and pilot valve 110 subject againto energization through thermostat contacts TI-IS.

The circuit shown in FIG. 3 is powered from a 12 volt D.C. source andutilizes a conventional inverter 123 to change the 12 volt DC. to l 10volt A.C. at secondary 16B to supply power to the flame detectioncircuit 30 previously described.

Considering now the operation of the circuit shown in FIG. 3, a 12 voltDC. signal applied to terminals 118 and 119 causes current to flowthrough warp switch contacts 112, warp switch heater 111 to sparkgenerator 11B and pilot valve 110 to ignite the pilot burner. Currentalso flows to inverter circuit 123 to generate 110 volt A.C. atsecondary 'l6B'to supply power to flame detector circuit 30.

The presence of pilot flame on electrode 31 causes SCR 39 to conduct aspreviously described, thereby causing the energization of the valve coil21 on main gas solenoid and causing main burner gas to flow and tobecome ignited by the pilot. Coil 107, also located on the main gasvalve solenoid, acts like the secondary winding of a transformer, coil21 being the primary, and supplies a voltage to disable the sparkgenerator 11B whenever the main gas valve is energized.

Considering the operation of the 12 volt D.C.- spark generator, the highvoltage transformer T1 has three windings, high voltage secondary 101,primary winding 100 and feedback winding 102.

Current flows into emitter of transistor 109 to colle c-. tor 121 and avoltage divider consisting of resistors 103 and 104. The voltage at thejunction of resistors 103 and 104 is sufiicient to cause current to flowthrough feedback winding 102, diode l13, base 116 to emitter 115 oftransistor 117 to 12 volt, terminal 119. As a result, transistor 117conducts and allows current to flow from 12 volt terminal 118 throughcontacts 112 and heater 111 through high voltage primary winding 100 toinduce a high voltage in the secondary winding 101 to produce sparks forignition at the electrodes.

As the current in the primary winding increases, feedback winding 102,located on the same magnetic core as primary winding 100, causestransistor 117 to conduct further until it becomes saturated. At thattime, the voltage induced into the feedback winding begins to decrease,thereby decreasing the conductivity of transistor 117 and the'current inprimary winding 100 decreasing to induce a negative voltage in thefeedback winding 102 to cut off the conduction of transistor 117 tocomplete one cycle of oscillation. The frequency of oscillation utilizedis approximately 1000 cycles per second.

After the spark ignites the pilot, sensing probe 31 causes SCR 39' toconduct to energize main gas valve solenoid coil 21 to turn on the maingas.

Energizing coil 21 induces a voltage in secondary winding 107 toincrease thevoltage on base 122 of transistor 109 sufliciently to cut itoff. This removes the bias on SCR 117 and disables the spark generator.If the pilot becomes extinguished for any reason, coil 21 becomesde-energized removing the voltage generated by coil 107 causingtransistor 109 to conduct to cause the spark generator to produce sparksfor re-ignition. If for any reason the pilot gas did not ignite, warpswitch heater actuates manual resettable contacts 112 tothe opencondition in approximately two minutes to de-energize the system. Warpswitch heater 111 normally does not actuate because one amp of currentdrawn by the spark circuit is eliminated during a normal operation. So,at start-up, warp switch heater current is approximately 1.3 amps andwarp switch heater current during a normal cycle is approximately 0.3

amp.

With reference to FIG. 4, the operation of the circuit shown is the sameas for the circuit of FIG. 3 except main valve coil 21 has been replacedby a relay coil 21 with contacts 125 and 124 for controlling main valveoperation and ignitor circuit 11C.

Contacts 124 energize the spark generating ignitor circuit 11C whichgenerates sparks in a manner previously described with respect to FIG.3, except that when pilot gas is ignited, probe 31 causes SCR 39 toconduct to energize relay 21 to opencontacts 124 to disable the ignitorcircuit 11C and closes contacts 125 to energize the main gas valve 126.

If the pilot flame is extinguished for any reason, probe 31 causes relaycoil 21 to de-energize to drop out the gas valve to shut off the mainburner gas and to activate the spark generator 11C. If afterapproximately two minutes the spark generator has not turned off, thewarp switch 111 opens its contacts 112 to de-energize the system, aspreviously described for FIG. 3.

I claim:

1. In an automatic fuel ignition system including a pilot source forestablishing a pilot flame and valve means operable when energized tosupply a gaseous fuel to burner apparatus, an electronic control circuitfor monitoring the pilot flame and controlling the energization of saidvalve means, said control circuit comprising pilot flame sensing meansincluding sensing electrode means located in the proximity of the pilotsource for sensing said 'pilot flame, and pulse generating meanscontrolled by said pilot flame sensing means to provide pulse outputs ofa first amplitude whenever the pilot flame is established and to providepulse outputs of a second amplitude whenever the pilot flame isextinguished, and switching means responsive to pulses at said firstamplitude to effect energization of said valve means, said switchingmeans being disabled whenever pulses at said second amplitude areprovided by said pulse generating means to cause deenergization of saidvalve means to prevent the flow of said gaseous fuel to the burnerapparatus whenever the pilot flame is extinguished.

2. An electronic control circuit as set forth in claim 1 whereinsaidpulse generating means includes a normally non-conducting controlledswitching device, first circuit means including capacitormeans connectedto an input electrode of said controlled switching device and meansconnected to a source of potential to provide a charging path for saidcapacitor means, discharge circuit means including said controlledswitching device for discharging said capacitor means whenever saidcontrolled switching device is rendered conductive, said pilot flamesensing means including second circuit means connected to a controlelectrode of said controlled switching device and operable toperiodically render said controlled switching device conductivepermitting said capacitor means to discharge over said discharge path toprovide said pulse outputs.

3. An electronic control circuit as set forth in claim 2 wherein saidsecond circuit means includes further capacitor means chargeable at afirst rate whenever the pilot flame is established to provide apotential at said control electrode to render said controlled switchingdevice conductive at a time when said capacitor means of said pulsegenerating means has charged to a first voltage and chargeable at asecond rate whenever the pilot flame is extinguished to provide apotential at said control electrode to render said controlled switchingdevice conductive at a time when said capacitor means of said pulsegenerating means has charged to a second voltage.

4. An electronic control circuit as set forth in claim 2, wherein saidpilot source includes a pilot valve means operable when energized tosupply pilot gas to an outlet, and pilot ignition means operable whenenergized to generate ignition sparks for igniting pilot gas emanatingfrom said outlet to establish a pilot flame, said normallynon-conducting controlled switching device being responsive to pulses atsaid first amplitude to effect the deenergization of said pilot ignitionmeans.

5. An electronic control circuit as set forth in claim 4, wherein saidswitching means includes a further controlled switching device which isnormally disabled and enabled by pulses at said first amplitude toeffect energization of said valve means. i

6. An electronic control circuit as set forth in claim 11, wherein said.valve means includes a solenoid operated valve having a main gas valvecoil and a secondary coil, said secondary coil being energized to causethe pilot ignition means to be disabled whenever said main gas valvecoil is energized.

7. Anelectronic control circuit as set forth in claim 5, wherein saidswitching means further includes normally deenergized relay means, saidrelay means being energized by said further controlled switching deviceto control the energization of said gas valve means whenever pulses ofsaid first amplitude are provided by said pulse generating means.

8. An electronic control circuit as set forth in claim 7, wherein saidrelay means is operable when energized to effect deenergization of saidpilot ignition means.

9. An electronic control circuit as set forth in claim 1, furtherincluding power means adapted to be coupled to a direct current sourceof power for supplying a DC power signal to said control circuit, andinverter means responsive to said DC power signal to provide alternatingcurrent signals for said control circuit.

10. In an automatic fuel ignition system, pilot ignition means operablewhen energized to generate ignition.

sparks for igniting pilot gas emanating from a pilot source to establisha pilot flame, pilot flame monitoring means including flame sensingmeans having sensing electrode means located adjacent said pilot sourcefor sensing said pilot flame and pulse generating means including acontrolled switching device having first and second control electrodes,first capacitor means connected to said first control electrode, secondcapacitor means connected to said second control electrode, means forpermitting said first capacitor means to charge to a predetermined valueto provide a first potential at said first control electrode, meanspermitting said second capacitor means to charge to a predeterminedvalue at a firstrate whenever said pilot flame is extinguished and at asecond rate whenever said pilot flame is established to provide a secondpotential at said second control electrode, said controlled switchingdevice being enabled whenever the potential difference between saidfirst and second control electrodes exceeds a predetermined value topermit said first capacitor means to discharge over said controlledswitching device to effect the generation of pulse outputs of a firstamplitude whenever the pilot flame is established and pulse outputs of asecond amplitude whenever the pilot flameis extinguished, switchingmeans responsive to pulses of said first amplitude to de-energize saidpilot ignition meansand to prepare an 'energizing'path for gas valvemeans which'supplies gas to'gas burner apparatus, said switching meansbeing'disa'bled whenever pulses of said second amplitude are "providedby said pulse generating means to interrupt said energizing path forsaid gas valve means and effect energization of said pilot ignitionmeans wlienever said'pilotflame is extinguished. 11

1 1. An electronic control circuit as set forth in claim wherein saidswitching means includes normally deenergized relaymeans having contactmeans connected in an energizing path for said valve means and anormally disabled controlled switching'device connected in series with'acoil of said relay means across a source of potential, said controlledswitching device of said switching means being enabled responsive topulses at said first amplitude to effect energization of said relaymeans to operate said contact'means to permit energization of said gas.valve. means.

12. An electronic circuit as set ,fort.hiin claim 11 wherein said relay,means includes further contact means connected inan-energizingcircuitfor said pilot ignition means. and operated wheneversaid relay means"for'normally connecting said pilot ignition meansisde-energizedtolpermitenergization of said a pilot ignition means.-...I

13. In anaut'ornatic fuel ignitionsystem including pilot source meansfor establishing a pilot flame and valve means operable when energizedto supply a gaseous fuel to burner apparatus, an electronic controlcircuit for monitoring the pilot flame and controlling the energizationof said valve means said control circuit comprising pilot flamemonitoring means having a controlled switching device, first circuitmeans including capacitor means connected to a source of cyclical ACvoltage and to a first control electrode of said controlled switchingdevice to permit said capacitor means to charge during each first halfcycle of the AC voltage thereby providing a potential at said firstcontrol electrode, second circuit means connected to said AC voltagesource and to a second control electrode of said controlled switchingdevice including sensing electrode means located in the proximity of thepilot source means for sensing the pilot flame such that said secondcircuit means provides a potential at said second control electrodeeffective to render said controlled switching device conductive at afirst time during each first half cycle of the AC voltage whenever thepilot flame is extinguished and to render said controlled switchingdevice conductive at a later time during each first half cycle of the ACvoltage whenever the pilot flame is established, output means forproviding a discharge path over said controlled switching device forsaid capacitor means whenever said controlled switching device isrendered conductive to provide a pulse output, and switching meansresponsive to the pulse output provided by said flame monitoring meanswhenever the pilot flame is established to prepare an energizing pathfor said valve means, said switching means being disabled to interruptsaid energizing path for said valve means whenever the pilot flame isextinguished, and said controlled switching device being maintainednon-conductive in the event of a component failure in said pilot flamemonitoring circuit means.

14. An electronic control circuit in a fuel ignition system as set forthin claim 13 wherein said pilot source means includes a pilot source andpilot ignition means operable when energized to generate ignition sparksfor means to'said AC voltage source to energize said pilot ignitionmeans whenever the pilot flame is extinguished, said relay means beingenergized to operate said contact means to thereby disconnect said pilotignition'rneans from said AC voltage source whenever the pilotflame isestablished.

An electronic control circuit in a fuel ignition system as set forth'inclaim '14 wherein said switching means further includes afurthercontrolled switching device connected in series with a coil ofsaid relay means to said AC voltage source, said further controlledswitching device being normally nonconductingand being renderedconductive responsive to the'pulse output of said flame monitoring meansto effect energization of said relay means during'e ach first half cycleof said AC voltage whenever the pilot'flame is e'stablished, said relaymeans having further contact means connected in an energizing circuitfor said valve means and operated whenever said relay means is energi zed to permit eriergization of said valve means.

16. Ah electronic control circuit is a fuel ignition system as set'forthin claim 15 wherein said switching means includes means for maintainingsaid relay means energizedduring each second half cycle of the ACvoltage. A A I 17. An electronic control circuit in a fuel ignitionsystem as set forth in claim 16 wherein said output means of said pilotflame monitoring means includes a pair of resistors connected inparallel between an output electrode of said controlled switching deviceand a point of reference potential for said AC voltage source.

18. In an automatic fuel ignition system including a pilot source forestablishing a pilot flame and valve means operable when energized tosupply a gaseous fuel to burner apparatus, an electronic control circuitfor monitoring the pilot flame and controlling the energization of saidvalve means comprising input means, said control circuit including firstand second conductor means for supplying a cyclical AC voltage to saidcontrol circuit, a normally non-conducting controlled switching device,first circuit means connected between said first and second conductormeans, said first circuit means including first capacitor meansconnected to a first control electrode of said controlled switchingdevice and chargeable during each first half cycle of said AC voltage toprovide a potential at said first control electrode, second circuitmeans including second capacitor means connected between a point ofreference potential and said second conductor means, means forconnecting a second control electrode of said controlled switchingdevice to said point of reference potential, sensing electrode means,and means for connecting said sensing electrode means to said firstconductor means, said sensing electrode means being located in theproximity of the pilot source and positioned in a spaced relationshipwith said point of reference potential to provide a gap therebetweenwhich is bridged by the pilot flame whenever the pilot flame isestablished such that a charging path for said second capacitor means isprovided between said first and second conductor means over said gap topermit said second capacitor means to charge at a first rate wheneverthe pilot flame is extinguished to thereby provide a potential at saidsecond control electrode and to permit said second capacitor means tocharge at a faster rate whenever the pilot flame is established andbridges. said gap to thereby provide a potential at said second controlelectrode, said controlled switching device being rendered conductivewhenever the potential difference between said first and second controlelectrodes exceeds a predetermined amount such that said controlledswitching device is rendered conductive at a first time during eachfirst half cycle of the AC voltage whenever the pilot flame isextinguished and at a later time during each first half cycle of the ACvoltage whenever the pilot flame is established, output means forpermitting said first capacitor means to discharge over said controlledswitching device whenever said controlled switching device is renderedconductive to effect the generation of a pulse output, and switchingmeans responsive to the pulse output provided over said output meanswhenever the pilot flame is established to prepare an energizing pathfor said valve means, said switching means being disabled to interruptsaid energizing path for said valve means whenever the pilot flame isextinguished.

19. An electronic control circuit as set forth in claim 18 wherein saidfirst circuit means includes means for limiting the potential at saidfirst control electrode of said controlled switching device. to apredetermined value. I

20. An electronic control circuit as set forth in claim 18 wherein saidsecond circuit means includes means connected between said secondcontrolled switching device and said second conductor means for limitingthe potential at said second control electrode.

21. An electronic control circuit as set forth in claim 18 wherein saidswitching means comprises normally de-energized relay means having anenergizing coil and contact means connected in the energizing path forsaid valve means and a further normally nonconductive controlledswitching device connected in series with said coil between said firstand second conductor means, said further controlled switching devicebeing rendered conductive by the pulse output provided over said outputmeans during each first half cycle of the AC voltage whenever the pilotflame is established to effect energization of said relay means tooperate said contact means.

22. An electronic control circuit as set-forth in claim 19 wherein saidfurther controlled switching device is rendered non-conductive duringeach second half cycle of the AC voltage and said switching meansfurther intained non-c onductive in the event of a component failure insaid pilot flame monitoring means.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 3,902,839

DATED September 2, 1975 INVENTOR( 1 Russell B. Matthews It is certifiedthat error appears in the above-identified patent and that said LettersPatent Q are hereby corrected as shown below:

Column 14, line 41, after "valve means" insert said control circuit line42, cancel "said control circuit".

Signed and Scaled this seventeenth Day of February 1976 r [SEAL] IAttest.

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ufPatemsand Trademarks

1. In an automatic fuel ignition system including a pilot source forestablishing a pilot flame and valve means operable when energized tosupply a gaseous fuel to burner apparatus, an electronic control circuitfor monitoring the pilot flame and controlling the energization of saidvalve means, said control circuit comprising pilot flame sensing meansincluding sensing electrode means located in the proximity of the pilotsource for sensing said pilot flame, and pulse generating meanscontrolled by said pilot flame sensing means to provide pulse outputs ofa first amplitude whenever the pilot flame is established and to providepulse outputs of a second amplitude whenever the pilot flame isextinguished, and switching means responsive to pulses at said firstamplitude to effect energization of said valve means, said switchingmeans being disabled whenever pulses at said second amplitude areprovided by said pulse generating means to cause deenergization of saidvalve means to prevent the flow of said gaseous fuel to the burnerapparatus whenever the pilot flame is extinguished.
 2. An electroniccontrol circuit as set forth in claim 1 wherein said pulse generatingmeans includes a normally non-conducting controlled switching device,first circuit means including capacitor means connected to an inputelectrode of said controlled switching device and means connected to asource of potential to provide a charging path for said capacitor means,discharge circuit means including said controlled switching device fordischarging said capacitor means whenever said controlled switchingdevice is rendered conductive, said pilot flame sensing means includingsecond circuit means connected to a control electrode of said controlledswitching device and operable to periodically render said controlledswitching device conductive permitting said capacitor means to dischargeover said discharge path to provide said pulse outputs.
 3. An electroniccontrol circuit as set forth in claim 2 wherein said second circuitmeans includes further capacitor means chargeable at a first ratewhenever the pilot flame is established to provide a potential at saidcontrol electrode to render said controlled switching device conductiveat a time when said capacitor means of said pulse generating means hascharged to a first voltage and chargeable at a second rate whenever thepilot flame is extinguished to provide a potential at said controlelectrode to render said controlled switching device conductive at atime when said capacitor means of said pulse generating means hascharged to a second voltage.
 4. An electronic control circuit as setforth in claim 2, wherein said pilot source includes a pilot valve meansoperable when energized to supply pilot gas to an outlet, and pilotignition means operable when energized to generate ignition sparks forigniting pilot gas emanating from said outlet to establish a pilotflame, said normally non-conducting controlled switching device beingresponsive to pulses at said first amplitude to effect thedeenergization of said pilot ignition means.
 5. An electronic controlcircuit as set forth in claim 4, wherein said switching means includes afurther controlled switching device which is normally disabled andenabled by pulses at said first amplitude to effect energization of saidvalve means.
 6. An electronic control circuit as set forth in claim 11,wherein said valve means includes a solenoid operated valve having amain gas valve coil and a secondary coil, said secondary coil beingenergized to cause the pilot ignition means to be disabled whenever saidmain gas valve coil is energized.
 7. An electronic control circuit asset forth in claim 5, wherein said switching means further includesnormally deenergized relay means, said relay means being energized bysaid further controlled switching device to control the energization ofsaid gas valve means whenever pulses of said first amplitude areprovided by said pulse generating means.
 8. An electronic controlcircuit as set forth in claim 7, wherein said relay means is operablewhen energized to effect deenergization of said pilot ignition means. 9.An electronic control circuit as set forth in claim 1, further includingpower means adapted to be coupled to a direct current source of powerfor supplying a DC power signal to said control circuit, and invertermeans responsive to said DC power signal to provide alternating currentsignals for said control circuit.
 10. In an automatic fuel ignitionsystem, pilot ignition means operable when energized to generateignition sparks for igniting pilot gas emanating from a pilot source toestablish a pilot flame, pilot flame monitoring means including flamesensing means having sensing electrode means located adjacent said pilotsource for sensing said pilot flame and pulse generating means includinga controlled switching device having first and second controlelectrodes, first capacitor means connected to said first controlelectrode, second capacitor means connected to said second controlelectrode, means for permitting said first capacitor means to charge toa predetermined value to provide a first potential at said first controlelectrode, means permitting said second capacitor means to charge to apredetermined value at a first rate whenever said pilot flame isextinguished and at a second rate whenever said pilot flame isestablished to provide a second potential at said second controlelectrode, said controlled switching device being enabled whenever thepotential difference between said first and second control electrodesexceeds a predetermined value to permit said first capacitor means todischarge over said controlled switching device to effect the generationof pulse outputs of a first amplitude whenever the pilot flame isestablished and pulse outputs of a second amplitude whenever the pilotflame is extinguished, switching means responsive to pulses of saidfirst amplitude to de-energize said pilot ignition means and to preparean energizing path for gas valve means which supplies gas to gas burnerapparatus, said switching means being disabled whenever pulses of saidsecond amplitude are provided by said pulse generating means tointerrupt said energizing path for said gas valve means and effectenergization of said pilot ignition means whenever said pilot flame isextinguished.
 11. An electronic control circuit as set forth in claim 10wherein said switching means includes normally de-energized relay meanshaving contact means connected in an enErgizing path for said valvemeans and a normally disabled controlled switching device connected inseries with a coil of said relay means across a source of potential,said controlled switching device of said switching means being enabledresponsive to pulses at said first amplitude to effect energization ofsaid relay means to operate said contact means to permit energization ofsaid gas valve means.
 12. An electronic circuit as set forth in claim 11wherein said relay means includes further contact means connected in anenergizing circuit for said pilot ignition means and operated wheneversaid relay means is de-energized to permit energization of said pilotignition means.
 13. In an automatic fuel ignition system including pilotsource means for establishing a pilot flame and valve means operablewhen energized to supply a gaseous fuel to burner apparatus, anelectronic control circuit for monitoring the pilot flame andcontrolling the energization of said valve means said control circuitcomprising pilot flame monitoring means having a controlled switchingdevice, first circuit means including capacitor means connected to asource of cyclical AC voltage and to a first control electrode of saidcontrolled switching device to permit said capacitor means to chargeduring each first half cycle of the AC voltage thereby providing apotential at said first control electrode, second circuit meansconnected to said AC voltage source and to a second control electrode ofsaid controlled switching device including sensing electrode meanslocated in the proximity of the pilot source means for sensing the pilotflame such that said second circuit means provides a potential at saidsecond control electrode effective to render said controlled switchingdevice conductive at a first time during each first half cycle of the ACvoltage whenever the pilot flame is extinguished and to render saidcontrolled switching device conductive at a later time during each firsthalf cycle of the AC voltage whenever the pilot flame is established,output means for providing a discharge path over said controlledswitching device for said capacitor means whenever said controlledswitching device is rendered conductive to provide a pulse output, andswitching means responsive to the pulse output provided by said flamemonitoring means whenever the pilot flame is established to prepare anenergizing path for said valve means, said switching means beingdisabled to interrupt said energizing path for said valve means wheneverthe pilot flame is extinguished, and said controlled switching devicebeing maintained non-conductive in the event of a component failure insaid pilot flame monitoring circuit means.
 14. An electronic controlcircuit in a fuel ignition system as set forth in claim 13 wherein saidpilot source means includes a pilot source and pilot ignition meansoperable when energized to generate ignition sparks for igniting pilotgas emanating from said pilot source to establish a pilot flame, saidswitching means including normally de-energized relay means havingcontact means for normally connecting said pilot ignition means to saidAC voltage source to energize said pilot ignition means whenever thepilot flame is extinguished, said relay means being energized to operatesaid contact means to thereby disconnect said pilot ignition means fromsaid AC voltage source whenever the pilot flame is established.
 15. Anelectronic control circuit in a fuel ignition system as set forth inclaim 14 wherein said switching means further includes a furthercontrolled switching device connected in series with a coil of saidrelay means to said AC voltage source, said further controlled switchingdevice being normally non-conducting and being rendered conductiveresponsive to the pulse output of said flame monitoring means to effectenergization of said relay means during each first half cycle of said ACvoltage whenever the pilot flame is established, said relay means havingfurther contAct means connected in an energizing circuit for said valvemeans and operated whenever said relay means is energized to permitenergization of said valve means.
 16. An electronic control circuit is afuel ignition system as set forth in claim 15 wherein said switchingmeans includes means for maintaining said relay means energized duringeach second half cycle of the AC voltage.
 17. An electronic controlcircuit in a fuel ignition system as set forth in claim 16 wherein saidoutput means of said pilot flame monitoring means includes a pair ofresistors connected in parallel between an output electrode of saidcontrolled switching device and a point of reference potential for saidAC voltage source.
 18. In an automatic fuel ignition system including apilot source for establishing a pilot flame and valve means operablewhen energized to supply a gaseous fuel to burner apparatus, anelectronic control circuit for monitoring the pilot flame andcontrolling the energization of said valve means comprising input means,said control circuit including first and second conductor means forsupplying a cyclical AC voltage to said control circuit, a normallynon-conducting controlled switching device, first circuit meansconnected between said first and second conductor means, said firstcircuit means including first capacitor means connected to a firstcontrol electrode of said controlled switching device and chargeableduring each first half cycle of said AC voltage to provide a potentialat said first control electrode, second circuit means including secondcapacitor means connected between a point of reference potential andsaid second conductor means, means for connecting a second controlelectrode of said controlled switching device to said point of referencepotential, sensing electrode means, and means for connecting saidsensing electrode means to said first conductor means, said sensingelectrode means being located in the proximity of the pilot source andpositioned in a spaced relationship with said point of referencepotential to provide a gap therebetween which is bridged by the pilotflame whenever the pilot flame is established such that a charging pathfor said second capacitor means is provided between said first andsecond conductor means over said gap to permit said second capacitormeans to charge at a first rate whenever the pilot flame is extinguishedto thereby provide a potential at said second control electrode and topermit said second capacitor means to charge at a faster rate wheneverthe pilot flame is established and bridges said gap to thereby provide apotential at said second control electrode, said controlled switchingdevice being rendered conductive whenever the potential differencebetween said first and second control electrodes exceeds a predeterminedamount such that said controlled switching device is rendered conductiveat a first time during each first half cycle of the AC voltage wheneverthe pilot flame is extinguished and at a later time during each firsthalf cycle of the AC voltage whenever the pilot flame is established,output means for permitting said first capacitor means to discharge oversaid controlled switching device whenever said controlled switchingdevice is rendered conductive to effect the generation of a pulseoutput, and switching means responsive to the pulse output provided oversaid output means whenever the pilot flame is established to prepare anenergizing path for said valve means, said switching means beingdisabled to interrupt said energizing path for said valve means wheneverthe pilot flame is extinguished.
 19. An electronic control circuit asset forth in claim 18 wherein said first circuit means includes meansfor limiting the potential at said first control electrode of saidcontrolled switching device to a predetermined value.
 20. An electroniccontrol circuit as set forth in claim 18 wherein said second circuitmeans includes means connected between said second controlled switchingdevIce and said second conductor means for limiting the potential atsaid second control electrode.
 21. An electronic control circuit as setforth in claim 18 wherein said switching means comprises normallyde-energized relay means having an energizing coil and contact meansconnected in the energizing path for said valve means and a furthernormally non-conductive controlled switching device connected in serieswith said coil between said first and second conductor means, saidfurther controlled switching device being rendered conductive by thepulse output provided over said output means during each first halfcycle of the AC voltage whenever the pilot flame is established toeffect energization of said relay means to operate said contact means.22. An electronic control circuit as set forth in claim 19 wherein saidfurther controlled switching device is rendered non-conductive duringeach second half cycle of the AC voltage and said switching meansfurther includes means for maintaining said relay means energized duringthe second half cycle of the AC voltage.
 23. An electronic controlcircuit as set forth in claim 3 wherein said controlled switching deviceis maintained non-conductive in the event of a component failure in saidpilot flame monitoring means.